Hydridorhodathiaboranes: Synthesis, Characterization, and Reactivity

The reaction between pyridine and [8,8-(PPh3)2-nido-8,7-RhSB9H10] (1) has given the opportunity to synthesize a new family of 11-vertex hydridorhodathiaboranes that feature boron-bound N-heterocyclic ligands. To explore the scope of this reaction, 1 has been treated with the methylpyridine isomers (picolines) 2-Me-NC5H4, 3-Me-NC5H4, and 4-Me-NC5H4, affording the picoline ligated clusters [8,8,8-(H)­(PPh3)2-9-(L)-nido-8,7-RhSB9H9], where L = 2-Me-NC5H4 (3), 3-Me-NC5H4 (4), 4-Me-NC5H4 (5). Thermal treatment of these nido clusters leads to dehydrogenation and the formation of isonido/closo-[1,1-(PPh3)2-3-(L)-1,2-RhSB9H8] (9–11). Compounds 3–5 react with ethylene to form [1,1-(η2-C2H4)­(PPh3)-3-(L)-1,2-RhSB9H8] (13–15). Similarly, treatment of 3–5 with carbon monoxide produces [1,1-(CO)­(PPh3)-3-(L)-1,2-RhSB9H8] (17–19). These series of η2-C2H4 and CO ligated 11-vertex isonido/closo-rhodathiaboranes result from the substitution of one PPh3 ligand by ethylene or CO together with H2 loss and a concomitant nido to closo/isonido cluster structural transformation. The reactivity of 3–5 with propene, 1-hexene, and cyclohexene under a hydrogen atmosphere is also reported and compared with the reactivity of the pyridine ligated analogue [8,8,8-(H)­(PPh3)2-9-(NC5H5)-nido-8,7-RhSB9H9] (2). Low-temperature NMR studies have allowed the characterization of intermediates which undergo inter- and intramolecular exchange processes, depending on the nature of the N-heterocyclic ligand. The CO ligand enhances the nonrigidity of the cluster, opening mechanisms of H2 loss.